Lithium ion secondary batteries have been gaining attention as a high capacity power source mainly for portable devices. Developments are still conducted currently, aiming for further high capacity lithium ion secondary batteries. Generally, lithium ion secondary batteries include a positive electrode using lithium cobalt oxide (LiCoO2) as the active material, and a negative electrode using various graphites as the active material. Between the positive electrode and the negative electrode, a separator (microporous resin film) which electronically insulates both electrodes and keeps electrolyte is disposed. For the resin film material, polyolefin is generally used.
The cut-off-voltage of charge of batteries including a positive electrode containing lithium cobalt oxide as the active material and a negative electrode containing graphite as the active material is generally set to 4.1 to 4.2 V. In such a case, all the lithium included in the positive electrode is not necessarily used for the battery reaction. Therefore, by setting the cut-off-voltage of charge further higher to increase the utilization rate of the lithium in lithium cobalt oxide, the energy density of lithium ion secondary batteries can be increased.
On the other hand, when the separator contacting the positive electrode surface comprises polyethylene, in a charged battery, deterioration of the separator due to oxidization occurs especially at high temperatures, which is known to cause battery deterioration.
Thus, to prevent the deterioration of the separator in a charged battery due to oxidation, there has been proposed that a layer comprising polypropylene, which is more resistant to oxidization than polyethylene, is provided at the side of the separator contacting the positive electrode (Japanese Laid-Open Patent Publication No. 2000-100408). For example, a separator in which a layer comprising polyethylene and a layer comprising polypropylene are stacked is proposed.
However, even with polypropylene which is more resistant to oxidization than polyethylene, when the cut-off-voltage of charge of the lithium ion secondary battery is set to 4.3 V or more, oxidization advances to deteriorate the battery.